Accept any scheduler driver entrypoint

We broke the possibility in Ocata with Icdcf839b6d28893694bfa1355e9dbe8dbb5ea8c3
to use other scheduler drivers but the ones we provided in tree.

Unfortunately, that was an incidental change without any communication.

Removing the choices kwarg will allow operators to run their own scheduler driver.
Whether Nova would stop supporting custom drivers would require a totally separate
change which would clearly communicate thru a deprecation notice but that is not
the intent for that bugfix, which aims only to bring back the capability.

Change-Id: I346881bc3bc48794b139cc471be1de11c49b8ee3
Closes-Bug: #1704788

nova/conf/scheduler.py

@@ -34,25 +34,32 @@
 """),
     cfg.StrOpt("driver",
         default="filter_scheduler",
-        choices=("filter_scheduler", "caching_scheduler",
-                 "chance_scheduler", "fake_scheduler"),
         deprecated_name="scheduler_driver",
         deprecated_group="DEFAULT",
         help="""
-The class of the driver used by the scheduler.
+The class of the driver used by the scheduler. This should be chosen from one
+of the entrypoints under the namespace 'nova.scheduler.driver' of file
+'setup.cfg'. If nothing is specified in this option, the 'filter_scheduler' is
+used.
 
-The options are chosen from the entry points under the namespace
-'nova.scheduler.driver' in 'setup.cfg'.
+Other options are:
+
+* 'caching_scheduler' which aggressively caches the system state for better
+  individual scheduler performance at the risk of more retries when running
+  multiple schedulers.
+* 'chance_scheduler' which simply picks a host at random.
+* 'fake_scheduler' which is used for testing.
 
 Possible values:
 
-* A string, where the string corresponds to the class name of a scheduler
-  driver. There are a number of options available:
-** 'caching_scheduler', which aggressively caches the system state for better
-   individual scheduler performance at the risk of more retries when running
-   multiple schedulers
-** 'chance_scheduler', which simply picks a host at random
-** 'fake_scheduler', which is used for testing
+* Any of the drivers included in Nova:
+** filter_scheduler
+** caching_scheduler
+** chance_scheduler
+** fake_scheduler
+* You may also set this to the entry point name of a custom scheduler driver,
+  but you will be responsible for creating and maintaining it in your setup.cfg
+  file.
 """),
     cfg.IntOpt("periodic_task_interval",
         default=60,

nova/tests/unit/scheduler/test_scheduler.py

@@ -67,6 +67,16 @@ def test_init_using_caching_schedulerdriver(self,
         driver = self.manager_cls().driver
         self.assertIsInstance(driver, caching_scheduler.CachingScheduler)
 
+    @mock.patch.object(host_manager.HostManager, '_init_instance_info')
+    @mock.patch.object(host_manager.HostManager, '_init_aggregates')
+    def test_init_nonexist_schedulerdriver(self,
+                                           mock_init_agg,
+                                           mock_init_inst):
+        self.flags(driver='nonexist_scheduler', group='scheduler')
+        # The entry point has to be defined in setup.cfg and nova-scheduler has
+        # to be deployed again before using a custom value.
+        self.assertRaises(RuntimeError, self.manager_cls)
+
 
 class SchedulerManagerTestCase(test.NoDBTestCase):
     """Test case for scheduler manager."""

releasenotes/notes/bug-1704788-490797827bae9142.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - Correctly allow the use of a custom scheduler driver by using the name of
+    the custom driver entry point in the ``[scheduler]/driver`` config option.
+    You must also update the entry point in ``setup.cfg``.